The field of the invention is magnetic resonance imaging (MRI) and, in particular, local coils for use in MRI of the wrist.
In MRI, a uniform magnetic field B0 is applied to an imaged object to align the object's nuclear spins along a z-axis. In response to a radio frequency (RF) excitation signal of the proper frequency oriented within the x-y plane, the nuclei precess about the z-axis.
In a typical imaging sequence for magnetic resonance imaging, the precessing nuclear spins are then phase and frequency encoded according to their positions, using magnetic gradient fields. A weak nuclear magnetic resonance generated by the precessing nuclei may then be sensed by the RF coil and recorded as an NMR signal. From this NMR signal for a series of such signal acquisitions with different phase encodings, a slice image may be derived according to well-known reconstruction techniques in which the phase and frequency encoding is used to map NMR signal strength to different locations in the image.
The RF excitation and the NMR signal may be transmitted and received respectively by means of one or more RF coils. Improvements in the signal to noise ratio of the received NMR signal can be obtained by placing “local coils” on the patient. The local coil having a smaller reception pattern can focus in on the region of interest to obtain a stronger signal and to receive less noise. Phased array RF coils are multiple loop local coils whose outputs are processed separately and are electrically independent.
Desirably, a local coil may be “wrapped” about a portion of the body so that the volume within the local coil conforms closely to the imaged part. One way of accomplishing this is the use of flexible coils having thin copper conductors held in fabric-like panels that may be literally wrapped about the patient.
Coils with rigid structure can also be used but for such coils the ability to have the local coil closely conform to the patient is limited by the ability of the patient to thread his or her limb into the coils structure. For this reason, it is known to construct coils, for example, for imaging the hand or wrist, in two parts that may be separated from each other along a seam line so that the hand may be inserted, and then reassembled using several clamps holding the two parts together about the limb. Such coils provide good support for the antenna structure and the patient's limb but can be cumbersome for the MRI operator who must contend with positioning the patient and manipulating the loose part of the coil into alignment and managing the clamps necessary to hold the pieces together.